Process for producing carbon-nanotube grafted substrate

ABSTRACT

The present invention relates to a process for producing a carbon nanotube-grafted substrate, the process comprising: providing a substrate having catalytic material deposited thereon; and synthesising carbon nanotubes on the substrate by a chemical vapour deposition process in a reaction chamber; characterised in that the process comprises providing a counter electrode, applying a potential difference to the substrate in relation to the counter electrode and maintaining the potential difference of the substrate in relation to the counter electrode during the chemical vapour deposition process.

The present invention relates to a process for producing a carbonnanotube (CNT)-grafted substrate.

BACKGROUND

The grafting of carbon nanotubes (CNTs) to a substrate, for example acarbon fibre material, can serve many functions including, for example,as a coating to protect against damage from abrasion, and compression.Carbon fibres (CFs) have wide applicability as components offibre-reinforced composite materials. The growth/grafting of CNTs ontoreinforcing CF surfaces can serve as an interface between the substrateand matrix material in a composite to improve composite structuralperformance, through improved interfacial bonding of the matrix and theCF reinforcement. Grafted CNTs improve compatibility with a resinmatrix, in terms of wetting and adhesion. Grafting of CNTs also providesopportunities not found with traditional sizings to enhance transverseproperties, both mechanical (strength, stiffness, etc) and functional(thermal/electrical conductivity, solvent resistance, etc).

Methods of incorporating CNTs into fibre reinforced matrices includeinfiltrating CNT-loaded resins and depositing pre-grown CNTs on fibresby wet processes such as coating of aqueous dispersions. Undesirably,however, these processes generally result in CNTs parallel to a primaryfibre surface. In the former method loading fractions which can beincorporated are limited to 2 wt % due to increase in viscosity andself-filtration. An alternative approach is growing CNTs onto a fibresurface. This latter method is generally carried out by chemical vapourdeposition, mitigates CNT loading issues. Chemical vapour deposition(CVD) is a process by which CNTs may be grown on a substrate. CVDinvolves preparing a substrate with a layer of transition metal catalystparticles. The substrate is heated to approximately 700° C. and exposedto a carbon feedstock gas (containing carbon monoxide or a hydrocarbonor hydroxyl-substituted hydrocarbon such as acetylene, ethylene, ethanoland methane) and a reductive gas (such as hydrogen, or ammonia) with aninert carrier gas (such as helium, nitrogen or argon). Decomposition ofthe carbon-feedstock gas at the surface of the catalyst particle by anucleation process leads to growth of CNTs.

Growth of CNTs on carbon-substrates is not as well investigated asgrowth on silica, alumina and metal substrates as it is inherently moredifficult to grow CNTs on a substrate which dissociates into thecatalyst. Dissociation of the substrate material can damage of thesubstrate structure. The synthesis of CNTs from a catalyst on a carbonsubstrate, leads to competition with the absorption of carbon of thesubstrate over the carbon feedstock and typically leads to pitting ofthe surface. Reactions between catalyst and substrate may also occurduring catalyst activation (usually under reducing gases), simpleheating, or due to interaction with synthesis by-products.

Prior art processes have utilised a barrier coating between a CFsubstrate and the synthesised CNTs (WO2011/053458), but this can providea weak interaction between the CNTs and the substrate. Typical barrierlayers comprise alumina, glass, alkoxysilane, methylsiloxane, or analumoxane. Some prior art processes, such as WO2011/053458, utilise aplasma-enhanced CVD (PE-CVD) process. The presence of a plasma altersCNT growth kinetics. PE-CVD results in increased CNT growth but normallywith a trade off with a poorer quality of CNTs synthesised. Typical CVDof aligned CNTs arrays on substrates produces CNTs in the order of tensof microns in length, which is undesirable in fibre composites due tothe need to pack fibres close together.

The present invention provides a process for grafting CNTs to asubstrate which avoids damage to the structure of the substrate, doesnot require barrier coating of the substrate, produces short, smalldiameter CNTs, can be used as a continuous process at atmosphericpressure, does not require generation of a plasma and which can be usedas part of an in line manufacturing process.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a process for producing acarbon nanotube-grafted substrate, the process comprising:

-   -   a. providing a substrate having catalytic material deposited        thereon; and    -   b. synthesising carbon nanotubes on the substrate by a chemical        vapour deposition process in a reaction chamber;        characterised in that the process comprises providing a counter        electrode, applying a potential difference to the substrate in        relation to the counter electrode and maintaining the potential        difference of the substrate in relation to the counter electrode        during the chemical vapour deposition process.

In a second aspect, the invention provides a carbon nanotube-graftedsubstrate produced by a process according to the first aspect of theinvention.

In a third aspect, the invention provides an apparatus for graftingcarbon nanotubes to a substrate, the apparatus comprising a reactionchamber, means for positioning a substrate having catalytic materialdeposited thereon in the reaction chamber, a counter electrode, meansfor applying a potential difference to the substrate in relation to thecounter electrode, heating means and means for exposing the substrate toa carbon feedstock gas and a reductive gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be put into practice in various ways and a number ofspecific embodiments will be described by way of example to illustratethe invention with reference to the accompanying figures, in which:

FIG. 1 shows a schematic diagram of a batch CVD set-up with potentialdifference applied to the substrate during CNT-synthesis and an overviewof the set up with arrangement of fibre substrates in frames andelectrical connections with a close up of the substrates held in frames.

FIG. 2 shows a circuit diagram for a set-up with potential differenceapplied to the substrate. The set-up has an amplifier, CF electrode onthe positive capacitor plate and graphite foil as the counter electrode.The inclusion of the switch ensures complete discharge from the set-upafter CNT growth.

FIG. 3.1 shows a scanning election microscope (SEM) image of batchcarbon nanotube-grafted carbon fibre with no potential differenceapplied to the substrate (batch CNT-g-CF (0V) and with an appliedpotential difference ((batch CNT-g-CF 300V)). SEM images (a) to (c) areCNT-g-CF synthesised without the application of a potential difference(0 V), (d) to (f) & transmission electron microscopy (TEM) images (g) &(h) are CNT-g-CF synthesised under an applied potential difference of300 V. FIG. 3.2 shows SEM images batch CNT-g-CF synthesised under anapplied potential difference of 300 V with the CF surface visible.

FIG. 4 shows a SEM image of CNT-g-CF synthesised under an appliedpotential difference of 300 V and bath sonicated for 5 min in ethanol(EtOH).

FIG. 5 shows Raman spectra comparing batch synthesized CNT-g-CF with andwithout potential difference applied to the substrate, with inserts ofintensity of G mode to intensity of D mode (I_(G) to I_(D)) ratio.

FIG. 6 shows single fibre tensile test data, namely tensile strength andtensile modulus, comparing batch synthesized CNT-g-CF with and withoutpotential difference applied to the substrate.

FIG. 7 shows a schematic of the continuous CVD set-up with concentricquartz tubes.

FIG. 8 shows a schematic of the various reaction zones determined by thequartz tube length (more clearly seen in FIG. 7) and pulling speed ofthe fibre tow through the furnace.

FIG. 9 shows a schematic of the electrode set-up used in the continuousCVD to syntheses continuous carbon nanotube-grafted-carbon fibre with anapplied potential difference of 300 V to the substrate (cont. CNT-g-CF(300V)).

FIG. 10 shows SEM images of cont. CNT-g-CF (0V).

FIG. 11 shows Raman spectra of CNT-g-CF synthesised using the continuousCVD set-up without application of a potential difference, with insertsof I_(G) to I_(D) ratio.

FIG. 12 shows SEM images of cont. CNT-g-CF (300V).

FIG. 13 shows an SEM enlarged area showing even CNT synthesis on CFsurface (cont. CNT-g-CF (300V)). Smaller images adjacent are TEM imagesconfirming few walled small diameter carbon nanotubes synthesised.

FIG. 14 shows Raman spectra of CNT-g-CF synthesised using the continuousCVD set-up with CF substrate under a potential difference (300V),inserts of I_(G) to I_(D) ratio included.

FIG. 15 shows single fibre tensile test data, namely tensile strengthand tensile modulus, comparing cont. CNT-g-CF with and without potentialdifference applied to the substrate.

FIG. 16 (a-c) shows SEM images of CNT-g-CFs produced by a process asdescribed herein, with increasing potential difference applied to thecarbon fibres.

DETAILED DESCRIPTION

The present disclosure is directed to a process as described herein forgrafting carbon nanotubes on a substrate by a chemical vapour deposition(CVD) process, wherein the substrate has a potential difference inrelation to a counter electrode during the CVD process. The substratemay be a fibre material, preferably a carbon fibre material or a carbonfibre precursor.

The meanings of terms used herein are explained below, and the presentinvention will be described in detail.

As used herein, a “fibre material” refers to a single fibre (filament)or any material comprising a plurality of fibres as the elementarystructural component, such as a multi-filament, yarn, tow, rod, panel,braid, ribbon, tape, woven or non-woven fabric, ply, mat, roving, ormixture thereof, and the like. A tow comprises a bundle of untwistedfilaments. A yarn comprises a bundle of twisted filaments. Tapes orribbons may comprise woven fibres or non-woven flattened tows. Primaryfibre materials may be assembled into fabric or sheet-like structures,woven fabrics and non-woven mats. Any of such fibre materials may serveas the substrate in the process of the invention.

As used herein, “carbon fibres” refer to fibres with a carbon content ofat least 92 wt %. Carbon fibres may be made from a polymeric precursor,pitch, or from carbon allotrope building blocks. Carbon fibres maycomprise a graphitic or non-graphitic structure.

Carbon fibres may be generated, for example, from a polymer startingmaterial such as polyacrylonitrile (PAN), which undergoes a processcomprising cyclization, oxidation, carbonization and optionallygraphitization to form carbon fibres. Carbon fibres may also begenerated from starting materials such as cellulose fibres, pitch,lignin, polyethylene, which undergo various processing steps, such asstabilization and oxidation, prior to carbonization and graphitization,to form carbon fibres. In the context of the invention described hereina carbon fibre precursor is a starting material as described above asmodified at any stage of processing to produce a carbon fibre, whereinit will be appreciated that a carbon fibre precursor is suitable for useas a substrate in the process of the invention if it is capable ofhaving a potential difference applied thereto, i.e. a carbon fibreprecursor that has undergone sufficient carbonization to be conductive.A carbon fibre precursor may therefore be a carbon-containing fibrematerial which is electrically conductive.

The processes described herein allow for the continuous production ofcarbon nanotubes of uniform length and distribution along spoolablelengths of tow, tapes, fabrics and other 3D woven structures. Whilevarious mats, woven and non-woven fabrics and the like can befunctionalized by processes of the invention, it is also possible togenerate such higher ordered structures from the parent tow, yarn or thelike after CNT functionalization of these parent materials. For example,a CNT-infused woven fabric can be generated from a CNT-infused carbonfibre tow.

As used herein, a material having “spoolable dimensions” refers to amaterial having at least one dimension that is not limited in length,allowing for the material to be stored on a spool or mandrel.

As used herein, the term “carbon nanotube” (CNT) refers to anycylindrically-shaped allotrope of sp² carbon including single-walledcarbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs),multi-walled carbon nanotubes (MWNTs). Carbon nanoforms associated withcarbon nanotubes include singlewalled, multiwalled, scrolled, full-core,herringbone, cupstacked, cone, platelets, toroid, spirals, coilednanoform, fibrils, ribbon, rods, or mixtures thereof. The process andapparatus of the invention are of particular use for growing MWNTs,wherein the MWNTs comprise graphitic layers substantially parallel tothe fibre axis.

As used herein, the term “grafted” means bonded and “grafting” means theprocess of bonding. Such bonding may involve direct covalent bonding,ionic bonding, pi-pi, and/or van der Waals force-mediated physisorptionof a mixture thereof. In the process of the invention, grafting of CNTsto a substrate is achieved by growing CNTs from catalyst deposited onthe substrate surface. The process of the invention as described hereindoes not require the provision of a barrier layer between the substrateand the grafted CNTs. Accordingly, the CNTs may be directly bonded tothe substrate or indirectly bonded through intervening residualcatalytic material (e.g. transition metal). Preferably, no materialother than catalytic material is present between the CNTs and thesubstrate.

As used herein, an “inerting gas” may be nitrogen, a group 18 noble gas,or a mixture thereof, preferably nitrogen, argon, helium or a mixturethereof.

As used herein, a “hydrocarbon” is an organic compound consistingentirely of hydrogen and carbon. A hydrocarbon may be straight chain,branched, cyclic, saturated, partially unsaturated, aromatic or anycombination thereof.

As used herein, a “nanomaterial” a materials with at least one dimensionin the size range up to 1000 nm, preferably up to 100 nm.

As used herein, a “graphite foil” is a sheet material comprisingcompacted graphite.

As used herein, an “electric field” as generated in the process orapparatus of the invention is an electric field (Vm⁻¹) between thesubstrate and the counter electrode, calculated on the basis of theequation:

Electric Field=V/z

In this equation, V is the voltage applied between the substrate and thecounter electrode and z is the minimum distance (m) between thesubstrate and the electrode in the set-up of the process or apparatus.

As used herein, the “power density” of a process or apparatus of theinvention is a power density (Wcm⁻³) calculated on the basis of theequation:

power density=current×voltage/volume

In this equation, current is the total current (A) in the capacitorcircuit of the process or apparatus, voltage (V) is the voltage appliedbetween the substrate and the counter electrode and volume is the volume(cm⁻³) enclosed by counter electrode.

As used herein, the “current density on the cross section of thesubstrate” is the current density (Acm⁻²) calculated on the basis of theequation:

current density=current/CSA

In this equation, current is the total current (A) in the capacitorcircuit of the process or apparatus and CSA is the cross sectional area(cm⁻²) of the substrate.

As used herein, the “current density on the counter electrode surfacearea” is the current density (Acm⁻²) calculated on the basis of theequation:

current density=current/counter electrode surface area

In this equation, current is the total current (A) in the capacitorcircuit of the process or apparatus and counter electrode surface areais the surface area of the surface of the counter electrode facing thesubstrate.

As used herein, “atmospheric pressure” is taken to be 1 bar (i.e.100,000 Pa). A pressure of no further than 0.5 bar either way fromatmospheric pressure equates to a pressure of about 50,000-150,000 Pa.

As used herein, the term “comprises” means “includes, but is not limitedto” any specified constituent component, process step or the like. Theterm “comprises” encompasses, without limitation, instances which“consist essentially of” any specified constituent component, processstep or the like.

In a first aspect, the present invention is directed to a process forproducing a carbon nanotube-grafted substrate, the process comprising:

-   -   a. providing a substrate having catalytic material deposited        thereon; and    -   b. synthesising carbon nanotubes on the substrate by a chemical        vapour deposition process in a reaction chamber;        characterised in that the process comprises providing a counter        electrode, applying a potential difference to the substrate in        relation to the counter electrode and maintaining the potential        difference of the substrate in relation to the counter electrode        during the chemical vapour deposition process.

The substrate is preferably a fibre material or a sheet material. In thecontext used herein, a fibre material (e.g. a carbon-containing fibrematerial) refers to a single fibre (filament) or any material comprisinga plurality of fibres as the elementary structural component, such as amulti-filament, yarn, tow, rod, panel, braid, ribbon, tapes, woven ornon-woven fabric, ply, mat, roving, or mixture thereof, and the like.Where the substrate comprises a plurality of fibres, the appliedpotential difference causes repulsion between the fibres andconsequently separation of the fibres, facilitating even growth of CNTson the fibres. This addresses uneven CNT deposition observed in theabsence of an applied potential difference, due to difficulty diffusinggases through dense fibre preforms and/or due to bald spots createdwhere fibres contact. A sheet material may comprise, for example, agraphite sheet, graphite foil, a graphene sheet, a metal sheet or thelike.

The CNTs may be grown directly on the substrate. Accordingly, thegrafting may be directly between the CNT and the substrate or depositedcatalytic material, with no barrier layer present therebetween. Typicalbarrier layers comprise, for example, alumina, glass; alkoxysilane,methylsiloxane, or an alumoxane. In the present invention, a barrierlayer (e.g. alumina, glass, alkoxysilane, methylsiloxane, or analumoxane) may be absent. The CNTs grow in an orientation substantiallyperpendicular to the longitudinal axis of the substrate.

The CNTs may be any type of CNT, for example multi-walled CNTs. Thediameter of CNTs grown may be from 0.1 to 1000 nm, from 1 to 500 nm orfrom 1 to 200 nm. The CNT diameter can be constant or vary along CNTlength. CNTs can be grown at various lengths, determined by residualtime in carbon nanotube CVD synthesis conditions. The CNT length may,for example, be from 5 nm to 10 mm, from 5 nm to 20 μm, from 50 nm to 1μm, or from 100 nm to 1 μm.

In the process of the invention a potential difference is applied to thesubstrate in relation to a counter electrode. Thus, the substrate may beconductive and comprise electrically conductive material. Accordingly,the substrate may be formed of an electrically conductive material whichis charged and acts as an electrode during the CVD process. In theprocess of the invention, the potential difference is maintained (i.e.remains present) during the CVD process. Charge may be applied directlyto the substrate by connection of the substrate to an electrical source.The substrate and the counter electrode may be arranged to form acapacitance circuit during operation of the CVD process, wherein thesubstrate and the counter electrode act as the two plates (electrodes)of a capacitor, respectively. During operation of the CVD process, thereis a potential difference between the two electrodes of the capacitor.The counter electrode may be of a configuration (e.g, tubular)surrounding the substrate, with the substrate positioned within thecounter electrode. The charge on the capacitor electrodes may slowlydischarge, as for capacitors in general. Ideally, current flow throughthe space between the electrodes is low and leakage of charge isminimised. To maintain a constant potential difference a low levelcurrent may be applied to the substrate.

As determined by cross sectional area of the substrate, in someembodiments the current density on the cross section of the substratedoes not exceed 50 Acm⁻², 25 Acm⁻², 10 Am⁻², 5 Acm⁻² or 3 Acm⁻². In someembodiments, the current density on the counter electrode surface areamay not exceed 10×10⁻⁴ Acm⁻², 6×10⁻⁴ Acm⁻², 10×10⁻⁵ Acm⁻², or 6×10⁻⁵Acm⁻².

In a process of the invention, the power density may preferably be 1Wcm⁻³ or less, 0.5 Wcm⁻³ or less, 0.2 Wcm⁻³ or less, or 0.05 Wcm⁻³ orless.

The substrate may be a carbon-containing substrate. Accordingly, whereinthe substrate is a fibre material as described above, it may be acarbon-containing fibre material, including a carbon fibre or a carbonfibre precursor, for example in the form of a tow. Wherein the substrateis a sheet material, the substrate may be a graphite sheet, graphitefoil, or a graphene sheet, or may comprise a plurality of graphite orgraphene sheets. A particular benefit of the process of the inventionwherein the substrate is a carbon-containing substrate is that theapplication of a potential difference to the substrate preventsdissociation of the underlying substrate structure, which is observedfor carbon-containing substrates in the absence of an applied potentialdifference.

In the process described herein, the substrate may comprise a partiallycarbonised PAN-precursor fibre. Partial carbonisation of precursor fibreshould be sufficient such that the fibre is conductive. The fibre may benon-graphitic, i.e. provided before final graphitisation.

In an alternative embodiment, the fibre material of the substrate maycomprise metal fibres.

The potential difference applied to the substrate in relation to thecounter electrode may be positive or negative. The potential differenceapplied to substrate may between 0.1 volts and 30 000 volts, preferablybetween 100 and 1000 volts, more preferably between 200 and 400 volts.Variation of the potential difference is avoided during the CVD process,with variation preferably kept within a range of 20, 10, 5 or 1 voltseither way of an applied potential difference. The potential differencemay be kept constant during the CVD process. The potential differencemay be applied to the substrate within the reaction chamber or outsidethe reaction chamber. This may be applied by connection of the substrateto an electrical power source, wherein connection may achieved, forexample by connection of a wire to the substrate and a power source, orby placing the substrate in contact with a power source to enable it tobecome charged before moving within or into the reaction chamber. Ifapplied outside the reaction chamber, the potential difference ismaintained within the reaction chamber, during the CVD process. Thesubstrate may be positive and the counter electrode negative. Thepotential difference may be applied at a level such that no plasma isgenerated during operation of the CVD process. The applied potentialdifference may generate an electric field of 1 Vμm⁻¹ or less, 0.5 Vμm⁻¹or less, 0.2 Vμm⁻¹ or less, 0.1 Vμm⁻¹ or less, or 0.07 Vμm⁻¹ or less.The electric field may be at least 0.01 Vμm⁻¹.

The counter electrode may be of a configuration (e.g, tubular)surrounding the substrate, with the substrate positioned within thecounter electrode. In this configuration, the counter electrode is openat two ends, enabling the substrate to be passed through the counterelectrode. The counter electrode may comprise graphite, for example agraphite foil, or a metal alloy. It will be appreciated that the metalalloy should be selected to be heat resistant and stable under the CVDsynthesis conditions employed.

The chemical vapour deposition process may comprise exposing thesubstrate to a reductive gas and a carbon feedstock gas at elevatedtemperature. The chemical vapour process is preferably carried out withthe addition of an inert carrier gas to give an otherwise inertatmosphere. The chemical vapour deposition process may be carried out ata temperature in the range of 400° C. to 1200° C., preferably at least600° C., e.g 650-850° C. Heating may be carried out by a furnace, e.g.by coil, rod, with the furnace optionally surrounding the reactionchamber. Heating may occur prior to application of a potentialdifference to the substrate. Other means of heating includeradiofrequency, microwave and resistive heating.

The reductive gas may comprise hydrogen. The reductive gas may beprovided as a mixture of the reductive gas with an inerting gas,preferably at a percentage of less than 15% by volume of reductive gasin inerting gas. This percentage is sufficiently low to exclude autoignition in mixtures of environmental gas at elevated temperatures.

The carbon feedstock gas may comprise a hydrocarbon,hydroxyl-substituted hydrocarbon or carbon monoxide, or a mixture of twoor more hydrocarbons, hydroxyl-substituted hydrocarbons or carbonmonoxide, in gaseous form. The hydrocarbon or hydroxyl-substitutedhydrocarbon is preferably a C₁₋₁₂, preferably a C₁₋₈ hydrocarbon orhydroxyl-substituted hydrocarbon. For example, the carbon feedstock gasmay comprise acetylene, ethanol, methane, ethane, ethylene, propane,toluene, xylene, carbon monoxide or a mixture thereof.

The carbon feedstock gas may be provided as a mixture with an inertinggas, preferably at a percentage of less than 15% by volume of reductivegas in inerting gas. This percentage is sufficiently low to exclude autoignition in mixtures of environmental gas at elevated temperatures.

The reaction chamber may comprise one or multiple gas injection points.The use of inerting gas limits environmental gas inclusion at openingsof the reactor, and in the central region or regions. Inerting gas alsoaids cooling of the substrate at the entrance/exit of the reactionchamber. The flow of reductive and carbon feedstock gases may be varied,independently, between 1 standard cubic centimetres per minute and 50litre per minute.

The process described herein does not require a low pressure/vacuumenvironment for the CVD reaction. Accordingly, the CVD reaction may becarried out at atmospheric pressure, or higher or lower than atmosphericpressure. Preferably the pressure is slightly higher or lower thanatmospheric pressure, e.g. no further than 0.5 bar either way.Preferably, the CVD reaction may be carried out at a pressure higherthan atmospheric pressure (e.g. up to 2 bar, or up to 1.5 bar). Thisaids in avoiding air being drawn into the reaction chamber.

The reaction chamber may have at least one opening to environmental gas,and can contain multiple openings.

The catalytic material, which is a CNT-forming catalytic material, maybe a d-block transition metal-containing catalytic material or anon-metal seed catalyst, or a mixture thereof.

Where the catalytic material is a d-block transition metal-containingcatalytic material it may comprise a d-block transition metal or analloy or non-alloy mixture of d-block transition metals, in elementalform, in salt form or as a coordination complex with an organic ligand,and mixtures thereof. Such salt forms include, without limitation,oxides, carbides, and nitrides. Non-limiting exemplary transition metalsinclude Ni, Fe, Co, Mo, Cu, Pt, Au, Pd, Y and Ag. The catalytic materialmay comprise, without limitation, iron (II) acetylacetone, iron (III)acetylacetone, nickel (II) acetylacetone, cobalt (II) acetylacetone,cobalt (III) acetylacetone, iron (III) nitrate, nickel (II) nitrate,cobalt (II) nitrate, cobalt (III) nitrate, or a mixture thereof.

Where the catalytic material is a non-metal seed catalytic material, itmay comprise silicon oxide, silicon, silicon carbide, germanium, carbon,diamond, amorphous carbon, aluminium oxide, zirconium oxide, carbonnanotubes or any other sp² carbon nanomaterial, or a mixture thereof.

The process may additionally comprise the step of forming the substratehaving catalytic material deposited thereon by depositing catalyticmaterial on a substrate material. Depositing may be carried out byapplying catalytic material in a solvent to a substrate material.Deposition may be carried out by a process such as, without limitation,spraying, dip coating, impregnation, incipient wetness or a combinationthereof.

For applying the catalytic material to the substrate, any solvent thatallows the catalytic material to be dispersed therein may be used. Suchsolvents can include, without limitation, ethanol, methanol, water,acetone, isopropanol, hexane, toluene, tetrahydrofuran (THF),cyclohexane, or a mixture thereof.

In an alternative approach, catalytic material may be deposited on asubstrate material by a gas phase deposition process, for example byexposing substrate to a gaseous source of metal such as ironpentacarbonyl.

The CVD process may be performed in a batchwise fashion.

Alternatively, the CVD process may be a continuous process. A CVDprocess performed in a continuous fashion, is one whereby CNTs aregrafted continuously along a spoolable length of substrate. The chemicalvapour deposition process may be a continuous process, wherein thesubstrate is moved through the reactor sequentially allowing reductionof carbon feedstock gas and synthesis of carbon nanotubes to occurin-situ, continuously along a length of the substrate.

The substrate may be moved through the reaction chamber at constantspeed, or allowed to dwell in a specific region or regions withinreaction chamber. The process may comprise providing substrate on aspool, wherein substrate is fed into the reaction chamber from the spooland, after passing through the reaction chamber, taken up on acollection spool. Movement of the substrate may be controlled by amechanical motor winder which may be tension controlled.

The residual time of any specific portion of the substrate within thereaction chamber may be between 0.1 and 10000 seconds. The residual timemay be at least 300 seconds or at least 900 seconds.

Deposition of catalytic material on the substrate may be continuouslyapplied in-line with continuous chemical vapour deposition reactor.After deposition and prior to the CVD process, the deposited catalyticmaterial can be treated, for example dried in inert conditions by afurnace, or infra-red radiative heater, in-line.

Continuous manufacturing of carbon nanotube grafted continuous substratemay proceed indefinitely, as long as enough source materials aresupplied to the reactor, and sooting pyrolysis of carbon feedstock isadequately controlled.

The process as described herein may comprise one or more steps offurther processing of the CNT-grafted substrate subsequent to CVD.Further processing steps may be applied for suitability of an intendedend use of the CNT-grafted substrate. The further processing maycomprise one or more of plasma treatment, surface roughening throughchemical or oxidative processes (e.g. carried out using potassiumhydroxide), application of a barrier coating, sizing, heat treatment,chemical functionalization, resin impregnation or impregnation withepoxy. Heat treatment may include graphitisation and/or carbonisation.This may be particularly beneficial to enhance strength of bonding ofthe CNTs to the substrate, particularly where the substrate is a carbonfibre material or carbon fibre precursor. A benefit of the process ofthe invention is that CVD to graft CNTs may be carried out as a stepwithin an inline production process, for example as one step within anin-line process for the production of CNT-grafted carbon fibres fromstarting materials.

Collection of carbon nanotube grafted continuous substrate may be madein environmental atmosphere, or in inerting regions, or mixture thereof.

In the process described herein, the substrate may be protected using acoating to prevent, damage, agglomeration of catalytic material, or itmay be used without a coating.

In a second aspect, the invention provides a carbon nanotube-graftedsubstrate produced by a process according to the first aspect of theinvention. The carbon-nanotube-grafted substrate may be for use as areinforcing element in composites, a catalyst support, an electrode, oran electromagnetic wave absorber/reflector.

In a third aspect, the present invention provides an apparatus forgrafting carbon nanotubes to a substrate, the apparatus comprising areaction chamber, means for positioning a substrate having catalyticmaterial deposited thereon in the reaction chamber, a counter electrode,means for applying a potential difference to the substrate in relationto the counter electrode, heating means and means for exposing thesubstrate to a carbon feedstock gas and a reductive gas. The apparatusmay be for performing a process according to the first aspect of theinvention as described herein.

In the apparatus, the substrate and the counter electrode may both beconnected to an electrical source, such that during operation acapacitance circuit is formed with the substrate acting as one electrodeof a capacitor and the counter electrode acting as the other electrodeof the capacitor.

The apparatus may comprise means for moving the substrate through thereaction chamber. This allows CNT grafting to occur as a continuousprocess. The means for moving the substrate through the reaction chambermay comprise a spool, creel or reel. The apparatus may comprise amechanical motor to drive movement of the substrate. The apparatus mayfurther comprise a means for gas exchange. The apparatus may comprisezones, with apertures present between the zones and means for supplyinga gas (gas inlets) to each zone. This enables a different gasenvironment to be provided in each zone.

The counter electrode may be a tubular component, for example formed ofa graphite foil. The heating means may be a furnace, for example a coil,rod or induction furnace or IR heater. The heating means may comprisemeans for application of a current to the substrate to provide directelectrical heating. The apparatus may further comprise a collectionspool for taking up substrate after passing through the reactionchamber. The apparatus may comprise a shielding means for shielding thesubstrate from the counter electrode. This may, for example, comprise aninsulating material, such as quartz, positioned between the substrateand the counter electrode.

The means for applying a potential difference to the substrate maycomprise a wire connection to an electrical source or a conductivemember (such as a metal pin) connected to a power source, whereby thesubstrate may be placed in contact with the conductive pin to enable itto become charged before moving within or into the reaction chamber.

The means for exposing the substrate to a carbon feed gas and areductive gas may comprise one or more gas inlets into the reactionchamber. The apparatus may further comprise one or more gas outlets forexit of gas from the reaction chamber.

The substrate, carbon feed gas, reductive gas and counter electrode maybe as defined in respect of the first aspect of the invention.

All features of each of the aspects of the invention as described abovecan be applied to the other aspects of the invention mutatis mutandis.

In the process of the invention, the potential is applied directly tothe substrate (e.g. carbon fibres) and this does not need to be carriedout under vacuum, in contrast to many plasma-enhanced CVD processes. Theelectrostatic field on a fibrous substrate spreads fibres due to mutualrepulsion. Significantly, everywhere in the substrate (e.g. a CF tow) isset to the same, well defined voltage. Spreading is useful as itprovides space for gas access and uniform CNT growth. The electrostaticcharge is also applied to the growing CNTs (and catalyst particles),causing them to repel on another, which helps their alignment andgrowth. The use of plasma CVD often grows defective nanotubes and canetch the growing material. In contrast, in the process of the inventionthermal CVD may be used. Running a process at near ambient pressureincreases processing convenience and ease of integration into otherprocess steps, for example in an in-line manufacturing process. Forexample, the process may be integrated with a process comprisingconventional PAN fibre processing.

The nanotubes grown by a process of the invention are small diameter andconveniently short (short enough not to reduce fibre packing fraction ina final composite use). The dimensions provide high surface area and mayoffer improved mechanical properties. In addition, fibre damage isavoided without a barrier layer, whilst enabling the growth of truemultiwalled nanotubes. Previous work either has tended to damage the CFproperties or produce less desirable herringbone or platelet nanofibresrather than MWNTs. The advantage of the current invention is likely toaccrue from the directly applied potential helping to maintain separatecatalyst particles.

An apparatus for performing a batch process of the invention is shown inFIG. 1. The apparatus comprises a reaction chamber 101 within which ispositioned a frame 102 holding the substrate 103, e.g. a CF electrode.In the illustrated embodiment, additional frames 104 are also presentfor holding control samples 105 (with no potential difference applied),although these additional frames and control samples are not essential.The substrate 103 and frame 102 are positioned within a shielding quartztube 106. This tube may be present between the counter electrode 107,e.g. a graphite foil, and the substrate 103 to prevent stray substratefibres discharging the circuit. Around the quartz tube 106 is locatedthe counter electrode 107, in the illustrated embodiment a tubulargraphite foil. Connections of the substrate 103 and the counterelectrode 107 to an electrical source are made by wires 108, 109,shielded by ceramic beads 110 to avoid shorting. The apparatus comprisesa heating source (furnace) 111 an inlet 112 for carbon feedstock andreductive gases and a gas exhaust/outlet 113. Connections to the circuitare denoted as 114. Control samples are denoted as 115. The arrowrepresents flow direction.

The apparatus shown in FIG. 1 forms a capacitance circuit duringoperation when a potential difference is applied, with the substrate 103and the counter electrode 107 acting as the two plates of the capacitor.FIG. 2 is a circuit diagram representing the capacitance circuit formed.

An apparatus for performing a continuous process of the invention isshown in FIGS. 7 and 8. The apparatus comprises concentric quartz tubes.Quartz outer tube 701 (51.0±0.5 OD, 2.5±0.2 wall thickness) supports theconcentric tubes, middle tube 702 (25.0±0.2 OD, 1.5±0.2 wall thickness)and inner tube 703 (19.0±0.3±0.5 OD, 1.5±0.2 wall thickness) in acantilever arrangement. The small dashes at the ends of the connectionsare gas inlet/outlet positions 704, 705. The apparatus is kept at roomenvironment. In the illustrated embodiment, 701 is a fixed outer 2″tube, 702 is a fixed middle tube, 703 is an inner tube with a variableposition. FIG. 8 shows a schematic of the various reaction zonesdetermined by the quartz tube length (more clearly seen in FIG. 7) andpulling speed of the fibre tow through the furnace 801. Substrate 802 ispulled though the multi-zone reactor from left to right, initiallyheating substrate 802 fibres in inert atmosphere to reaction temperature(zone C_(left) 803), reduction of catalytic material then occurs atreaction temperature in zone B 804, and CVD CNT-synthesis occurs in zoneA 805. Finally cooling of substrate occurs in an inert atmosphere (zoneC_(right) 806) and CNT-grafted substrate is collected at roomtemperature. In the illustrated embodiment, Zone C 803 and 806 arewithin an inert gas sleeve. The arrows in FIGS. 7 and 8 represent thedirection of substrate feed through the reaction chamber.

EXAMPLES

The examples show the improved synthesis of carbon nanotubes (CNTs) oncarbon fibre (CF) process in batch and continuous conditions in chemicalvapour deposition (CVD) under the application of a potential differenceto the substrate during synthesis. The descriptions of embodiments aremerely illustrative of the presented invention and deviations of theembodiments can be devised by those skilled in the art without departingfrom the scope of the invention. It therefore is intended that any suchvariations to be included within the scope of claims provided.

Example 1: Potential Applied Bias Synthesis of Carbon Nanotubes onCarbon Fibre in Batch Chemical Vapour Deposition (CVD) Synthesis

In the following example, when a potential difference (bias) was appliedto a carbon fibre substrate a significant increase in carbon nanotubes(CNTs) were synthesized and damage to the parent carbon fibre (CF) wasreduced. Throughout the following example, carbonnanotube-grafted-carbon fibre is referred to as CNT-g-CF.

CF substrate, (AS4C-GP-12K-8, HS-CP-4000, continuous tow,polyacrylonitrile (PAN)-based, Hexcel Composites, UK) were impregnatedwith catalytic material by submerging for 2 min in a 5 wt % bi-catalystmaterial comprising ron(III) nitrate (98% ACS reagent, Sigma-Aldrich,UK) and nickel(II) acetylacetonate (98%, VWR, UK) in ethanol solution(EtOH >99.7% BDH Prolabo, VWR, UK) in 1:1 stoichiometry, dip washed indeionised water (18Ω) for 1 min, then dried at standard ambientatmospheric temperature and pressure. All chemicals were usedas-received, CFs were not treated (virgin) prior to catalyst precursordeposition.

Once dried, the CF were placed inside the apparatus as shown in FIG. 1.Thermal CVD CNT-g-CF synthesis was carried out on 10 cm length of CF towwhich has been pre-deposited with catalyst precursor in a hot-walled CVDset-up using a 2″ quartz tubular furnace (PTF 15//610, Lenton, UK).Quartz frames (Robson Scientific, UK) were used to hold the fibres inposition in the reaction chamber (a furnace tube) maintainingaccessibility to gas flow. Graphite foil (GF), 99.8%, (C1179, AdventResearch Materials Ltd, UK) was chosen as the counter electrode in thecircuit as it is a conductive and flexible material which does notcatalyse the growth of CNTs. A sheet of compacted GF (approx.100×180×0.2 mm) used as-received, was rolled into a cylinder andinserted into the 2″ quartz tube, where it was allowed to unroll tocreate a 2″ ID tubular counter electrode. Electrical connection to thecounter electrode was made by piercing the GF with stainless steel (SS)wire. The electrical connection to CF was made by wrapping a piece of GFto the quartz frame then sandwiching the CFs with SS wire binding.

The wires inside the reaction chamber were kept apart by ceramic beadsthreaded over the wires to avoid shorting during the CNT-synthesis(white ceramic fish spine beads 1.5 mm bore, RS Components Ltd,Northants, UK). Quartz frames serve only to hold specimens in placeduring synthesis, and a shielding piece of quartz prevented shortingbetween the electrodes. The circuit arrangement is as shown in FIG. 2.Potential differences were measured using a high voltage probe (TestecHigh Voltage Probe TT-HVP 40, 1000:1 divider, division ratio accuracy1%) connected to a voltmeter (ISO-Tech IDM67, ±0.7% voltage, IEC 1010-1CAT II 600V). A 1:250 voltage amplifier (MM3P1.5/12, 1.5 W, 12 V input,linear high voltage output 3 kV max and 0.5 mA max, efficiency 55% to70%, Spellman High Voltage Electronics, UK) was used to increase thevoltage output in conjunction with a variable current and voltage powersupply source (Mastech HY3003D, variable D.C. supply, 30 V max and 3 Amax output, Digimess Instruments Ltd, UK). The connection to a powersupply to provide a potential difference was made through shielded wiresand a bespoke electrical feed through (LewVac Components Limited, UK)and reaction carried out within a 2″ quartz tube.

Samples were subjected to inert gas (argon) for purging the systemsprior to heating and then subjected to reducing atmosphere (argon with10 volume percentage hydrogen) and to carbon feedstock (acetylene) oncereaction temperature was obtained, as described in Table 1. Theexperimental procedure, including all relevant gas flows and typicalfurnace temperatures as measured by an external thermal probe, is setout in Table 1. For comparison, control samples were used to establishrelative differences between samples which had CNTs not grown under anapplied potential difference, in which the same CNT synthesistemperatures, gases and procedures were carried out (Table 1) but notattached to an electrical circuit.

Potential difference was applied/discharged to the CF substrate underinert gas conditions only. The charge imparted on the CF substrate was+300 V in relation to the counter electrode, with the counter electrodegraphite foil earthed (electric-field determined to be on the order of0.05 Vμm⁻¹). The opposite configuration was tested, CF substrate earthedand GF +300 V, with identical CNT morphology observed on CF.

TABLE 1 Gas with flow rate [standard Potential cubic centimetres]difference Ar + H₂ Measured applied Duration (10 vol % temperatureVoltage Current Time [min] Ar H₂) C₂H₂ [° C.] [V] ‡ [A] Furnaceconnections are sealed and checked for leaks. Electric circuitconstructed and checked. System is purged with inert gas prior toheating of furnace 10:00-10:20 20 500 — —    18° C. @ — — 10:0010:20-10:40 20 1000 — —    18° C. @ — — 10:20 Furnace switched on atheating rate 10° C. min⁻¹ to 770° C. under inert gas flow 10:40-12:00 80500 — —    18° C. @ — — 10:40 Once at 770° C. switched on power supplyand applied potential difference 12:00-12:10 10 2000 — —   777° C. @ 3000.01 12:00 Introduction of reductive gas for 10 min prior to carbonfeed-stock 12:10-12:20 10 — 2000 —   772° C. @ 300 0.01 12:1012:20-13:20 60 — 2000 10   769° C. @ 300 0.01 12:20 Once reaction iscomplete, furnace is switched off and left to cool with inert gas flow13:20-13:30 10 500 — — ~680° C. @ 300 0.01 13:20 Potential differenceswitched off, any static discharged, continue to cool with inert gasflow 13:30-~18:30 ~230 — — — ~600° C. @ 0 0 13:30 ~18:30-next — — — —~100° C. @ — — day 18:30 Furnace left sealed with inert gas (notflowing) until cooled down to room temperature. Furnace is opened thefollowing day to remove samples and for cleaning. N.B. the circuit isswitched on and off in inert atmosphere

Samples were removed at room temperature and microscopy (FIGS. 3 and 4),Raman (FIG. 5), mechanical (FIG. 6) analysis undertaken. FIGS. 3 and 4show SEM and TEM images of batch synthesised carbon nanotube graftedcarbon fibre with no potential difference (batch CNT-g-CF (0V)) andbatch synthesised carbon nanotube grafted carbon fibre which has had theapplication of a potential difference (batch CNT-g-CF (300V)), showingsignificant increase in CNTs grown from the CF surface in a dense forestlike synthesis, and no observable damage associated with the growth ofCNTs. The synthesis of CNT-g-CF with no applied potential difference(0V), FIG. 3.1 (a) to (c), shows severe pitting on the CF surface withsporadic CNT growth. CF subjected to a potential difference during CNTsynthesis (300V), FIG. 3.1 (d) to (f), showed significant increase inthe number of CNTs grown from the CF surface, showing a dense forestlike synthesis. In areas where CNTs had been pulled away during SEMpreparation of CNT-g-CF (300V) there was no observable damage associatedwith the growth of CNTs on the CF surface (FIG. 3.2). TEM images (FIGS.3.1 (g) & (h)) of the synthesised CNT-g-CF showed nanotube formationshad grown from the surface of the CF in a random orientation. The CNTssynthesised under application of a potential difference had an average56 nm diameter (standard deviation 35.7, range 8 to 155 nm), and whereisolated CNT-g-CF are observed diameters of the fibre averaged 26 μm(standard deviation 7.8, range 14 to 47 μm), indicating a (average)perpendicular CNT growth length of approx. 10 μm. CNT perpendicularthickness is calculated assuming that bi-catalyst precursor deposited CFdiameter after CNT synthesis is unchanged (6.9 μm, standard deviation0.2, range 6.4 to 7.1 μm, N.B. Hexcel data sheet for as-received AS4Cfibre diameter 6.9 μm.

The diameter of the fibres as received and after bi-catalyst depositionwere measured and both were 6.9 μm in diameter as per the product datasheet.

Raman analysis of CNT-g-CF provides a non-destructive method of probingglobal sample properties. The I_(G) to I_(D) ration is often provided toestablish the damage, and infer the structure of the CNTs on the CF. CFsubstrate provides an inherent coinciding addition to the measured Ramanintensity, and care must be taken to establish if damage to theunderlying substrate or the synthesised CNTs are adequately represented.Raman spectroscopy was carried out on LabRAM Infinity with 532 nm [2.33eV] Nd-YAG green laser (LabSpec V4.18-06, 2005 software interface,Horiba Jobin Yvon Ltd., UK) in a backscattered geometry. Subtle modesharpening of the D and G bands can be used to evaluate the presence ofCNTs.

FIG. 5 shows Raman spectra of various fibres with inserts of I_(G) toI_(D) ratio. Spectra shown are an average of 5, taken over all sampleareas with the exception of batch CNT-g-CF (0V) samples, which onlyareas of CNT growth were selected due to high variation and low CNTcoverage on the surface. Batch CNT-g-CF (300V) refers to CNTs graftedonto the fibre surface with the application of a potential differenceduring CNT-synthesis. Batch CNT-g-CF (0V) is a typical CNT-g-CFsynthesised sample carried out without the application of a potentialdifference in identical CVD conditions (N.B. 60 min CNT-synthesisduration). Batch CNT-g-CF (300V) shows distinct D and G mode sharpeningattributed to the CNTs grown on the surface of the graphitic CF, whencompared to the as-received CF and bi-catalyst precursor deposited CFspectra. A lowering of the I_(D)/I_(G) ratio for CNT-g-CF (300 V)suggests that the CNTs grown are relatively defective, as observed inTEM analysis (FIG. 3.1 (h)), and typical of CVD MWNTs. The increasedintensity in the D band for CNT-g-CF (300 V) was attributed to theaddition of CNTs and not to any damage potentially sustained to theunderlying parent structure (CF). This conclusion was rationalisedbecause CNT-g-CF retained as-received CF tensile strength, observed inmechanical analysis and considering tensile strength is sensitive tosurface defects. No mode sharpening of the D and G was observed in batchCNT-g-CF (0V) indicating that the relative density of CNTs on thesurface is limited. The reduction in I_(D)/I_(G) observed for theCNT-g-CF (0 V) is associated with pitting of the catalyst particles intothe CF and sparsely synthesised CNTs, both clearly evident frommicroscopy analysis (FIG. 3.1 (a)), which directly correlate to thereduced single fibre mechanical properties observed.

Tensile strength (with generated tensile strength values using theWeibull shape and scalar parameters for gauge dependence) and tensilemodulus of elasticity (Tensile Modulus) were measured for batch CNT-g-CF(300V), batch CNT-g-CF (0V) and bi-catalyst deposited CF, prior to CVD.Measurements were taken according to BS ISO 11566 standard (BritishStandards Institution, Carbon fibre—Determination of the TensileProperties of Single-Filament Specimens. ISO 11566, (1996)), Method B, Kvalue 16.6 mm/N, Epoxy adhesive 50/50 hardener to resin (Araldite RapidAdhesive, Bostik Findley Ltd., Leicester, UK), cross sectional areavalue taken from data sheet (Hexcel Composites, HexTow™ AS4 CarbonFiber, Data Sheet, 2009), 15 μm crosshead speed, standard ambienttemperature and pressure (SATP). Results of this mechanical analysis areshown in FIG. 6. These results demonstrate that the batch CNT-g-CF (0V)has damage to the underlying CF support, showing a reduction in overalltensile properties associated with damage to the surface. The modulus ofbatch CNT-g-CF (0V) is reduced which also indicates that damage hasoccurred to the core of the fibres. In the batch CNT-g-CF (300V) samplethe tensile strength and modulus are closely matched to the bi-catalystdeposited samples indicating limited damage has occurred during thesynthesis and retention of substrate properties. The retention of thefibre properties is significant for uses as reinforcement in compositesand for basic handling of the hierarchical material. To illustrate theadhesion between the CF and CNTs is mechanically robust the synthesisedCNT-g-CF were bath sonicated in EtOH as a qualitative experiment. A fourcm section was cut from the CNT-g-CF (300V) tow, left to soak inhigh-performance liquid chromatography (HPLC) EtOH (5 ml) for 1 h thenbath sonicated in a small vial for 1 h (temperature 30° C.), theresultant fibres are shown in FIG. 4. The CNTs surrounding CFs are shownto be bound to the fibre surface after agitation with the CNT forestcollapsed and densified due to solvent drying. CNT adhesion clearlydemonstrates that the bond between CF and CNTs is significantly strong,an advantage over silica and alumina fibres where the CNT-fibre bond islikely to be weak.

Determination of specific surface area was achieved throughre-arrangement of the BET (Brunauer, Emmett and Teller) isotherm inaccordance with BSI ISO 9277. Determination of specific surface area ofsolids by gas adsorption-BET method 2012:24 using Micromeritics TriStarSurface Area and Porosity Analyser and TriStar3000 6.07 software(Micromeritics UK Ltd., UK) with oxygen-free N₂ (99.998 vol %, BOC, UK).Prior to a measurement, samples were degassed in N₂ for at least 4 h at80° C. Specific surface area analysis showed the characteristically lowCF, 0.28 m²g⁻¹ was not significantly increased by the deposition of thebi-catalyst precursor, but showed a marked increase with the grafting ofCNTs to the surface through the application of a potential difference.The BET measurement requires a significantly large volume of low surfacematerial to produce a measureable specific surface area as the CNT-g-CF(0V) was produced on a significantly small scale (on the order of 10 cmof fibre processed) the sample was not of significant quantity norsurface area to provide a reliable surface area value. In contrast theCNT-g-CF (300V) had a significant surface area contribution from thegrafted CNTs. CF without CNT showed a Type I adsorption isotherm, theCNT-g-CF (300 V) showed a Type IV adsorption isotherm.

In summary, these results obtained indicate that CF substrate is damagedin batch CNT-grafted-CF (0V) CVD synthesis, however when a potentialdifference is applied during synthesis batch CNT-grafted-CF (300V) showsretention of tensile properties. Raman shows no mode sharpening on thesamples without applied potential difference indicating a very low CNTcoverage, but distinctive CNT sharpening in batch CNT-grafted-CF (300V)sample indicating high CNT presence and confirming the SEM analysis.

Electric Field Determination

The electric field (E-field) generated can be calculated by making anumber of simplifications. Firstly, instead of treating the carbonfibres separately (a roving typically contains 12000 fibres in total)they are treated as a single large fibre (radius=r), held inside alarger radius cylinder (R where r<R), comparable to a cylindricalcapacitor (coaxial) arrangement. E-field is assumed to be uniform andperpendicular from the surface of the rod, towards the outer cylinderand that both surfaces are closed, infinitely long and separated by adistance (r−R=z). The space between the electrodes is filled with Ar andthe dielectric constant of Ar taken as equal to air=vacuum=1. If a smallregion of the rod and cylinder are taken, such that the arrangement isnow reduced to a parallel plate configuration, then the problem isreduced to the well-known E-field between two simple parallel plates;

${E\mspace{14mu}{field}} = \begin{matrix}V \\z\end{matrix}$E-field = electric  field  (V  m⁻¹), V = voltage  (V), z = distance  from  plates  (m)

The E-field value provided is that at the point with shortest distancebetween electrodes (relating to the highest E-field).

The effective E-field strength when a potential difference of 300 V isapplied to the batch arrangement described above is 0.05 Vμm⁻¹.

Example 2: Continuous Development of Potential Applied Bias Synthesis ofCarbon Nanotubes on Carbon Fibre

In the following example, under application of potential difference evencarbon nanotube synthesis was observed on carbon substrates in allpositions, with smaller diameter carbon nanotubes. Without a potentialdifference, limited carbon nanotube synthesis which is not uniform alongfibre length was observed.

CF substrate, in this instance AS4C-GP 12K-8 (HS-CP-4000, HexcelComposites, UK) deposited with catalysts (nickel(II) acetylacetonate andiron(III) nitrate nonahydrate in 50:50 ratio at 4 weight percentage insolution (ethanol, then washed in deionized water) suitable for thesynthesis of CNTs, once dried were threaded inside the apparatus with awhole spooled tow acting as a substrate source, as shown in FIGS. 7 and8. FIG. 7 shows a schematic of the continuous CVD set-up with concentricquartz tubes. Quartz outer tube (51.0±0.5 OD, 2.5±0.2 wall thickness)supports the concentric tubes, middle tube (25.0±0.2 OD, 1.5±0.2 wallthickness) and inner tube (19.0±0.3±0.5 OD, 1.5±0.2 wall thickness) in acantilever arrangement through the custom designed quick connection.Quartz tubes sourced from Robson Scientific. The small dashes at theends of the quick fit connections are gas inlet/out positions. Furnaceis 1.2 m long with a 0.8 m stable temperature hot zone, with glassware(end to end) in total measuring up to 3 m. FIG. 8 shows a schematic ofthe various reaction zones determined by the quartz tube length (moreclearly seen in FIG. 7) and pulling speed of 1.2 m/h of the fibre towthrough the furnace. Fibre is pulled though the multi-zone reactor fromleft to right, initially heating fibres in inert atmosphere to reactiontemperature (zone C_(left)), reduction of catalyst precursor then occursat reaction temperature for 10 min (zone B), and CVD CNT-synthesis for30 min (zone A), finally cooling in an inert atmosphere (zone C_(right))and sample collection at room temperature.

The continuous set-up was purged with nitrogen prior to switching onfurnace and pre-deposited catalyst precursor loaded CF was heldstatically during heating to 770° C. at 10° C. min⁻¹, under nitrogenflow in the inert gas sleeves (flow rate 7500 standard cubic centimetres(sccm)). Once the furnace had achieved reaction temperature (770° C.)the initially heated fibre section was discarded and the pull-throughrate was set to 1.2 mh⁻¹. CNT-synthesis gases were then added to thereaction chamber, with fibre exposed to the reduction zone (N₂+H₂ (2.4volume percentage) at flow rate 3500 sccm) for 10 min, and CNT synthesiszone (N₂+C₂H₂ (1.3 volume percentage) at flow rate 325 sccm) for 30 minat the pull-through rate (1.2 mh⁻¹). The fibre was allowed to passthrough the continuous set-up for 2 h. The system was then cooled downto 80° C. under nitrogen flow in the inert gas sleeves only (flow rate7500 sccm) with the fibre held statically. Samples were takenperiodically along the fibre tow for SEM and for Raman spectralanalysis, only sample areas which had passed fully through the reactorwere chosen to demonstrate a consistent and typical overview of thecont. CNT-g-CF, with samples spanning a distance of at least 2 m intotal. The only difference between the continuous set-up without/withapplication of potential difference (cont. CNT-g-CF (0V) and cont.CNT-g-CF (300V), respectively), was the application of a potentialdifference of 300 V. FIG. 9 shows a schematic of the electrode set-upused in the continuous CVD to synthesize cont. CNT-g-CF (300V). Carbonfibre tow is shown being pulled through an insulating shielding quartztube to separate the fibres from the counter electrode (graphite foil)which is connected by being pierced by a wire (wire shielded withceramic beads to prevent shorting). The synthesis of CNTs occurs in thegrowth region of the shown in (zone A) whilst the counter electrode issituated in the inert atmosphere (zone C_(right)), and the connectionmade to the carbon fibres made before entry into the reactor (in thisinstance on the left). The connection to a power supply to provide apotential difference was made through rolling the CF over a stainlesssteel pin before entry to the reactor, with the counter electrode placedin-between the 2″ quartz tube and inner tube. The potential differencewas only applied/discharged under inert conditions and was kept constantat 300 V (CF positively charged).

The SEM images of FIG. 10 show various areas of CNT-g-CF grown in thecontinuous set-up without application of potential difference (cont.CNT-g-CF (0V)). Letters indicate on the schematic the relative positionof samples along fibre after it has fully passed through the reactionchamber ((a) is the pre-deposited catalyst precursor loaded CF which hasnot be subjected to CVD conditions and is included as a reference pointonly). All fibres have been deposited with iron nitrate and nickel(II)acetylacetonate bi-catalyst, sourced from a complete tow sample takenfrom a spool of over 25 m of fibre. There is poor uniformity ofsynthesised CNTs, in instances patchy CNT growth was noticed inlocalised areas (FIG. 10 (b)) and along the fibre tow there were largebare regions void of CNTs (FIGS. 10 (c) and (d)).

Raman spectra of cont. CNT-g-CF (0V) showed no mode sharpening, normallyan indication of CNTs over a graphitic substrate (CF signal). FIG. 11shows Raman spectra of CNT-g-CF as received (prior to catalystdeposition) and synthesised using the continuous CVD set-up with insertsof I_(G) to I_(D) ratio. Spectra shown are an average of 5, taken overall sample areas, with regions A-D denoting the position relative to SEMimages of FIG. 10. Region A is the Raman spectrum taken after thedeposition of iron nitrate and nickel(II) acetylacetonate (not subjectedto CVD conditions). Region inside furnace does not have a correspondingimage in FIG. 10, but shows a similar CNT grafting to other samplesshown, Regions B, C and D have passed fully through the CVD reactor, ata speed of 1.2 mh⁻¹.

The SEM images of FIG. 12 show cont. CNT-g-CF grown using the continuousset-up with substrate under a potential difference of 300V (cont.CNT-g-CF (300V)). Letters indicate on the schematic the relativeposition of samples along fibre after it has fully passed through thereaction chamber ((a) is the pre-deposited catalyst precursor loaded CFwhich has not be subjected to CVD conditions and is included as areference point only). All fibres have been deposited with iron nitrateand nickel(II) acetylacetonate bi-catalyst, sourced from a complete towsample taken from a spool of over 25 m of fibre. All regions observedhad an even growth of CNTs promoted though the application of apotential difference to the substrate (CF).

FIG. 13 shows an SEM enlarged area showing even CNT synthesis on CFsurface (cont. CNT-g-CF (300V)). The CNT produced were of small diameterand demonstrated a dense network. Smaller images adjacent are TEM imagesconfirming few walled small diameter carbon nanotubes synthesised. TheCNTs synthesised under application of a potential difference incontinuous CVD had an average 29 nm diameter (standard deviation 3.63,range 18 to 33 nm), isolated cont. CNT-g-CF (300 V) are observed to havefibre diameters averaged 7.0 μm (standard deviation 0.19, range 6.7 to7.3 μm). TEM analysis of the cont. CNT-g-CF (300 V) surface with a CNTforest thickness is 125 nm (standard deviation 68.9, range 272 to 20nm).

FIG. 14 shows Raman spectra of cont. CNT-g-CF (300V), with inserts ofI_(G) to I_(D) ratio included. Spectra shown are an average of 5, takenover all sample areas, with regions denoting the position relative toSEM images of FIG. 12. Regions B, C and D have passed fully through theCVD reactor, at a speed of 1.2 mh⁻¹. CF substrate was applied with apotential difference of 300 V during CVD compared to the counterelectrode (CF positively charged). Raman analysis of cont. CNT-g-CF(300V) with the observation of mode sharpening suggests that the fibreis significantly grafted with CNTs (Region C and Region D). In Regions Cand D the inclusion of the G′ mode is indicative of the addition ofrelatively crystalline CNTs to the CF surface.

Tensile strength (with generated tensile strength values using theWeibull shape and scalar parameters for gauge dependence) and tensilemodulus of elasticity (Tensile Modulus) were measured for cont. CNT-g-CF(300V), cont. CNT-g-CF (0V) and iron nitrate and nickel(II)acetylacetonate deposited CF, prior to CVD. Measurements were takenaccording to BS ISO 11566 standard, Method B, K value 16.6 mm/N, Epoxyadhesive 50/50 hardener to resin (Araldite Rapid Adhesive, BostikFindley Ltd., Leicester, UK), CSA value taken from data sheet (HexcelComposites, HexTow™ AS4 Carbon Fiber, Data Sheet, 2009), 15 μm crossheadspeed, standard ambient temperature and pressure (SATP). The results ofthis analysis are shown in FIG. 15. These results demonstrate that thecont. CNT-g-CF (0V) has damaged the underlying CF support and shows areduction in overall tensile strength associated with damage to thesurface. Reduction in tensile strength in cont. CNT-g-CF (0V) is not assignificant as in the batch CVD case (Batch CNT-g-CF (0V)), but there isdiminished CNT coverage, as determined through SEM and Raman analysis.The modulus in cont. CNT-g-CF (0V) and cont. CNT-g-CF (300V) is similarwhich suggests that no significant damage has occurred to the core ofthe fibre in either case. In the cont. CNT-g-CF (300V) sample tensilestrength closely matches the bi-catalyst deposited samples indicatinglimited damage has occurred during the synthesis with a low dependenceon gauge length, but a significant promotion of CNTs grafted on thesurface on the fibre was observed by SEM and Raman analysis.

The results obtained indicate that CNT synthesis without the applicationof potential difference shows hardly any CNT growth indicated by SEM andRaman (no disenable difference from the as-received and bi-catalyst) anda reduction of mechanical properties over as-received. CNT synthesiswith application of potential difference showed even coverage of smalldiameter CNTs grown along all fibres which passed through the growthzone with improved mechanical properties over non biased (cont. CNT-g-CF(0V)) growth. For continuous CNT-grafted-CF (300V) inclusion of the G′mode is indicative of the addition of crystalline CNTs to the CFsurface. Two metres of cont. CNT-g-CF (300V) on 12K AS4C tow wassuccessfully synthesized as proof of principle.

To reinforce findings, using the CVD continuous set-up and catalystloaded carbon fibres described previously, the potential differenceapplied to the carbon fibre was increased during CVD synthesis ofCNT-grafted-CF. The resultant CNT-g-CF (varying potential differencesapplied, FIGS. 16(a)-(c)) indicates that with an increasing potentialdifference between electrodes, CNT synthesis is promoted on fibresurfaces. The specified voltages (indicated in FIGS. 16(a)-(c)) relateto the system used and may be varied accordingly, without restriction ofthe general process as described here within.

Embodiments of the invention have been described by way of example only.It should be appreciated that variations of the described embodimentsmay be made without departing from the spirit and scope of theinvention.

1-25. (canceled)
 26. An apparatus for grafting carbon nanotubes to asubstrate, the apparatus comprising a reaction chamber, means forpositioning in the reaction chamber a substrate having catalyticmaterial deposited thereon, a counter electrode, means for applying apotential difference to the substrate in relation to the counterelectrode, heating means for heating the substrate in the reactionchamber and means for exposing the heated substrate to a carbonfeedstock gas and a reductive gas while the potential is applied. 27.The apparatus according to claim 26, further comprising an electricalsource configured to be connected to the substrate, wherein the counterelectrode is connected to the electrical source, such that duringoperation a capacitance circuit is formed with the substrate acting asone electrode of a capacitor and the counter electrode acting as theother electrode of the capacitor.
 28. The apparatus according to claim26, comprising means for moving the substrate through the reactionchamber. 29-31. (canceled)
 32. The apparatus according to claim 28,wherein the means for moving the substrate through the reaction chambercomprises a reel.
 33. The apparatus according to claim 26, furthercomprising a means for shielding the substrate from the counterelectrode.
 34. The apparatus according to claim 26, wherein thesubstrate comprises a carbon-containing material.
 35. The apparatusaccording to claim 26, wherein the substrate comprises a fibre material.36. The apparatus according to claim 26, wherein the substrate comprisesa carbon fibre material or precursor thereof.
 37. The apparatusaccording to claim 26, wherein the substrate is conductive.
 38. Theapparatus according to claim 26, wherein the carbon nanotubes aremulti-walled carbon nanotubes.
 39. The apparatus according to claim 26,wherein the reductive gas comprises hydrogen.
 40. The apparatusaccording to claim 26, wherein the carbon feedstock gas comprises carbonmonoxide, a hydrocarbon or hydroxyl-substituted hydrocarbon, or amixture of two or more of carbon monoxide, hydrocarbons orhydroxyl-substituted hydrocarbons, in gaseous form.
 41. The apparatusaccording to claim 26, wherein the catalytic material is a d-blocktransition metal-containing catalytic material or a non-metal seedcatalyst, or a mixture thereof.
 42. The apparatus according to claim 26,wherein the catalytic material comprises iron (II) acetylacetone, iron(III) acetylacetone, nickel (II) acetylacetone, cobalt (II)acetylacetone, cobalt (III) acetylacetone, iron (III) nitrate, nickel(II) nitrate, cobalt (II) nitrate, cobalt (III) nitrate, or a mixturethereof.
 43. The apparatus according to claim 26, wherein the catalyticmaterial comprises silicon oxide, silicon, silicon carbide, germanium,carbon, diamond, amorphous carbon aluminium oxide, zirconium oxide,carbon nanotubes or any other sp² carbon nanomaterial, or a mixturethereof.
 44. The apparatus according to claim 26, wherein the heatingmeans is configured to heat the substrate in the reaction chamber to atemperature in the range of 400° C. to 1200° C.
 45. The apparatusaccording to claim 26, configured such that the application of thepotential difference does not create a plasma.
 46. The apparatusaccording to claim 26, configured such that (a) the applied potentialdifference generates an electric field not exceeding 1 Vμm⁻¹; and/or (b)the current density on the cross section of the substrate does notexceed 50 Acm⁻²; and/or (c) the current density on the counter electrodesurface area does not exceed 10×10⁻⁴ Acm⁻²; and/or (d) the power densitydoes not exceed 1 Wcm⁻³.
 47. The apparatus according to claim 26,wherein the reaction chamber comprises a plurality of sequential zones,an aperture between each pair of adjacent zones, and means for supplyinggas to each zone.
 48. The apparatus according to claim 47, wherein theplurality of zones comprise a reduction zone having an inlet coupled toa source of the reductive gas, in which the heated substrate is exposedto the reductive gas; and a synthesis zone having an inlet coupled to asource of the carbon feedstock gas, in which the heated substrate isexposed to the carbon feedstock gas, wherein the apparatus is configuredto move a position on the heated substrate through the synthesis zoneafter moving the position on the heated substrate through the reductionzone.